Plant Diversity I: Non-vascular vs. vascular plants

Lecture #5
Plant Diversity I:
Non-vascular plants
&
Seedless Vascular plants
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1.2 billion years ago (BYA) – appearance of cyanobacteria on land
500 million years ago (MYA) – appearance of plants, fungi and animals
more than 290,000 known plant species today
plants inhabit all but the harshest environments
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many plants have returned to their aquatic “roots”
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such as some mountaintops, deserts areas and polar regions
e.g. some species of sea grasses
most present-day plants are terrestrial
presence of plants has enabled other life forms to survive on land
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through their production of O2
Plantae
Charophyta
Chlorophyta
Ancestral eukaryote
Plants
Charophyceans
Red algae
(Viridiplantae)
Chlorophytes
Animalia
Metazoans
(Opisthokonta)
Choanoflagellates
– similarities with algae:
• multicellular
• photosynthetic autotrophs
• cell walls with cellulose
• chlorophylls a and b
Fungi
– closest relatives are located with the clade
Charaophycea
– these share a common ancestor with the clade
Chlorophyta – include the green algae
Fungi
• evolution of plants proposed from algae
Archaeplastida
Rhodophyta
Plants and Algae
Unikonta
4 key traits of plants
– four key traits of plants (and charophyceans)
• provided by not only morphologic evidence but genetic evidence
• 1. rose-shaped complexes for cellulose synthesis – both charophyceans and land
plants have rosette cellulose-synthesizing complexes
– protein arrays found in the plasma membrane that synthesize cellulose microfibrils of the
cell wall
– plants and charophyceans have a higher percentage of cellulose in their cell walls than the
chlorophytans
• 2. peroxisome enzymes – peroxisomes have enzymes that help minimize the loss of
organic production as a result of photorespiration
• 3. flagellated sperm structure – similar to the charophyceans
• 4. formation of a phragmoplast – group of microtubules that forms between the
daughter nuclei of the dividing plant cell during mitosis
– synthesis of a new cell plate occures - divides the two daughter cells
Rosettes
Adaptations by Land plants
• advantages of terrestrial life:
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movement onto land would require protection of the zygote from drying out
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development of layer of durable polymer called sporopellenin – prevents exposed zygote from dessication
movement onto land resulted in the development of adaptations– facilitated survival and
reproduction on land
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stronger exposure to sunlight for photosynthesis
atmosphere offered more CO2 for photosynthesis
soil rich in nutrients
initially relatively few herbivores
e.g. development of a structural system to withstand the forces of gravity
e.g. changes adapting to the relative scarcity of water
these adaptations have defined the plant kingdom
but what adaptations are unique to plants?
depends on how you draw the boundary separating plants from algae
some traits related to terrestrial life
– for the earliest land plants – mycorrhizal associations with fungi for nutrient absorption
– epidermis with a waxy covering called a cuticle
– production of secondary compounds that are products of secondary metabolic pathways
• primary metabolic paths produce lipids, carbohydrates, amino acids – not unique to plants
– secondary paths produce compounds such as tannins, terpenes and alkaloids (defense against
herbivores and parasites), plus phenolics (flavonoids – absorb UV radiation, deter attacks by
pathogenic microbes)
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Kingdom Plantae contains the plants called embryophytes – plants with embryos
however, current debate advises some changes – 2 options:
– Kingdom Streptophytae – Embryophytes (land plates) + Charophyceans
OR
– Kingdom Viridiplantae – Embryophytes + Charophyceans + Chlorophytes
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botanists do not use the term phyla when classifying the plant kingdom – use divisions
currently accepted organization: development of two lineages or divisions: non-vascular
and vascular (390 MYA) – called the Bryophyta and Tracheophyta
– vascular lineage developed into the seedless vascular and seed vascular plants (360 MYA)
– seed vascular plants developed into the gymnosperms and angiosperms (130 MYA)
Viridiplantae
Streptophyta
** plants can be divided into 2
major categories
1. non-vascular
2. vascular – subdivided into 2
more categories:
a. seedless
b. seed
Plantae
Red algae
Chlorophytes Charophyceans Embryophytes
Ancestral alga
Land plants:
characteristics
• 4 key derived traits found in plants:
– 1. alternation of generations & multicellular,
dependent embryos
– 2. walled spores produced in sporangia
– 3. multicellular gametangia
– 4. apical meristems
Land plants: 4 characteristics
– 1. alternation of generations: alternation between multicellular haploid and diploid
stages in a life cycle
• seen also in some chlorophytans – but not in the charophyceans
• must be multicellular!!
• alternates between the gametophyte (haploid) and sporophyte (diploid)
• diploid sporophyte (mature plant) produces haploid spores via meiosis
• mitotic division of the haploid spore produces a multicellular gametophyte
(reproductive organ) which is still haploid!!
• the gametophyte produces haploid gametes by mitosis
• gametes fuse via syngamy to produce the zygote
• zygote grows via mitosis to develop a new sporophyte
• in ferns (non-vascular plants) – the sporophyte and gametophyte have distinct
phenotypic appearances – but they are forms of the same species
• in vascular plants – the gametophyte is microscopic
Haploid multicellular
organism (gametophyte)
-sporophytes – multicellular, diploid,
produce spores via meiosis
-gametophytes – multicellular, haploid,
produce gametes via mitosis
Mitosis
Mitosis
Gametes
Spores
MEIOSIS
FERTILIZATION
Zygote
Diploid multicellular
organism (sporophyte)
Mitosis
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1. Alternation of generations and
multicellular dependent embryos cont….
– in a life cycle with alternation of
generations – the multicellular embryos
develop from zygotes retained within
the female gametophyte
– maternal tissue provides nutrients
– embryos are called embryophytes
– embryo receives nutrition during
development from placental transfer
cells
• development of elaborate ingrowths of
the plasma membrane and cell wall of
the embryo
• lined with transfer cells
• enhance the transfer of nutrients from
parent to embryo
• analogous to the placental relationship
to the mather in eutherian animals
Multicellular,
Dependent Embryos
Embryo
Maternal
tissue
2 µm
10 µm
Wall
ingrowths
Placental
transfer
cell (blue line)
Land plants: 4 characteristics
– 3. walled spores in sporangia
• developing within the diploid sporophyte are multicellular organs called sporangia
(singular = sporangium) – production of haploid spores via meiosis
• within a sporangium are diploid cells called sporocytes or spore mother cells –
undergo meiosis to generate the haploid spores of the sporangium
• the spores are protected by sporoporellin – key adaptation to terrestrial life
– 4. multicellular gametogangia
• the haploid gametophyte undergoes production of gametes within multicellular
gametogania (singular = gametoganium)
• the production of gametes is through mitotic division
• female gametogania = archegonium - produces a single egg
• male gametogania = antheridium – produces many flagellated sperm
Walled Spores
Produced in Sporangia
Multicellular
Gametangia
Archegonium
with egg
Longitudinal section of
Sphagnum sporangium (LM)
Female gametophyte
Spores
Sporangium
Sporophyte
Male gametophyte
Gametophyte
Sporophyte and sporangium
of Sphagnum (a moss)
Archegonia and antheridia
of Marchantia (a liverwort)
Antheridium
with sperm
Land plants: characteristics
– 4. apical meristems
• light and CO2 are available above ground, water and minerals are found
mainly in the soil
• must be a way of collecting these components
• plants do this by growing in length – through the production of stems and
roots
• apical meristem – localized regions of cell division located at the tips of
shoots and roots
• e.g. shoot apical meristem – cells divide by mitosis and cytokinesis to
produce progenitor cells for the rest of the stem
• progenitor cells are the source for the tissues of the stem and root:
– protoderm (epidermis, cork),
– provascular tissue (xylem and phloem, vascular cambium)
– ground meristem (pith and cortex)
Apical
Meristem
of shoot
Shoot
Developing
leaves
Root
Plant Diversification
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plant fossils dating back to 475 MYA
one major way to distinguish groups of plants is to classify them as: vascular & nonvascular
– vascular tissue – extensive system formed by cells joined into tubes
– conduct water and nutrients
– those without these tubes – non-vascular plants
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bryophytes: term used to refer to all non-vascular plants
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do not form a monophyletic group or clade
known popularly as the mosses, liverworts and hornworts
debate as to how they are related to each other
don’t possess the advanced adaptations of vascular plants (e.g. roots & leaves)
they do share many characteristic with vascular plants – see the slide on plant
characteristics
vascular plants: clade that includes 93% of all surviving plant species
– categorized into smaller clades:
• 1. lycophytes – club mosses
• 2. pteryophytes – ferns
• 3. gymnosperms
• 4. angiosperms
Land plants
Vascular plants
Seed plants
Angiosperms
Gymnosperms
Pterophytes
Seedless vascular plants
Lycophytes
Mosses
Hornworts
Liverworts
Charophyceans
Bryophytes
Origin of seed plants
(about 360 mya)
Origin of vascular plants
(about 420 mya)
Origin of land plants
(about 475 mya)
Ancestral
green alga
Non-vascular plants
• commonly known as the bryophytes
– even though Bryophyta is one of the 3 phyla in this group
Plagiochila deltoidea = liverwort
• three phyla –
– 1. Phylum Hepatophyta: liverworts
• gametophytes are flattened into a thalloid or a leafy shape
Marchantia polymorpha = moss
– 2. Phylum Anthocerophyta – hornworts
• sporophyte can grow quite tall – sporangium along the length
– 3. Phylum Bryophyta – mosses
• mosses are not to be confused with the vascular mosses – lycophytes
• life cycle is dominated by the gametophyte stage
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gamete forming stage
gametophyte is only a few cells thick
anchored to the ground by rhizoids – long tubular single cells
NOT roots – not composed of tissues (cells only), lack specialized conducting
cells and are not responsible for water and mineral absorption
• some mosses are NOT mosses at all – Irish moss (red seaweed), reindeer
moss (lichen), club mosses (seedless vascular plant)
General Life Cycle: The Gametophyte
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dominant stage in these three phlya
when bryophyte spores land on favorable habitats – germinate and grow into gametophytes
the spore develops into a threadlike protonema – covers a large surface area for absorption or water
and minerals
each protonema produces a bud with an apical meristem (stem-cell like tissue for growth)
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the AM generates gamete-producing structures known as gametophores
gametophore bears the male or female gametangia
the protonema + gametophore = gametophyte
at the tip of the gametophore develops the reproductive structures = gametangia (singular =
gametangium)
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multiple gametangia develop on each plant
some bryophyte gametangia are bisexual – both antheridium and archegonium on the same gametophyte =
monoecious
most mosses have separate antheridium and archegonium located on separate gametophytes - dioecious
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“male and female” gametophytes
production of the gametes by mitosis since the gametophyte is haploid already
Gametophore of
female gametophyte
antheridium
500 µm
Foot
Seta
Sporangium
archegonium
Marchantia polymorpha,
a “thalloid” liverwort
Marchantia sporophyte (LM)
General Life Cycle: The Sporophyte
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in the plant kingdom – the sporophyte is the mature plant
fertilization is followed by development of the embryo within the archegonium
the embryo grows into a small sporophyte (diploid) - remains attached to the archegonium via a foot
– for absorption of nutrients
the sporophyte grows in length upward to produces a seta (stalk)
at the tip of this stalk forms a sporangium surrounded by a capsule
haploid spores develop in this sporangium via meiosis
in most mosses the upper part of the capsule forms a peristome – for gradual spore discharge
when the capsule matures – the peristome “pops” off and the spores are dispersed
from these spores comes new protonemata (singular = protonema)
hornwort and moss sporophytes tend to be large and more complicated
their sporophytes also have specialized pores = stomata
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support photosynthesis by allowing the exchange of CO2 and O2
also allow for the evaporation of water
also found in vascular plants
sporophyte
Life Cycle of a Moss
Raindrop
Key
Male
gametophyte
Haploid (n)
Diploid (2n)
Sperm
“Bud”
Spores develop into
threadlike protonemata.
A sperm swims
through a film of
moisture to an
archegonium and
fertilizes the egg.
Antheridia
The haploid
protonemata produce
“buds” that grow into
gametophytes.
Protonemata
Most mosses have separate male and
female gametophytes, with antheridia and
archegonia, respectively.
“Bud”
Egg
Gametophore
Spores
Female
gametophyte
Meiosis occurs and haploid spores develop
in the sporangium of the sporophyte. When the
sporangium lid pops off, the peristome “teeth”
regulate gradual release of the spores.
Peristome
Archegonia
Rhizoid
The sporophyte grows a long stalk, or
seta, that emerges from the archegonium.
Sporangium
MEIOSIS
Seta
Mature
sporophytes
Capsule
(sporangium)
Calyptra
Foot
FERTILIZATION
(within archegonium)
Zygote
Embryo
Archegonium
Young
sporophyte
Capsule with
peristome (SEM)
Female
gametophytes
Attached by its foot, the sporophyte
remains nutritionally dependent on the
gametophyte.
The diploid zygote develops
into a sporophyte embryo within
the archegonium.
http://www.sumanasinc.co
m/webcontent/animations/
content/moss.html
Moss Life Cycle
The Economics of Moss
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mosses have very lightweight spores
easy distribution has allowed for the establishment of
mosses around the globe
very common and diverse in moist forests and wetlands
can help retain nitrogen in the soil
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many species harbor cyanobacteria that increase the
availability of nitrogen to the moss
many species can survive drought and rehydrate when
moisture reappears
one wetland moss = Sphagnum or “peat moss”
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peat moss = partially decayed remnants of Sphagnum
major component of partially decayed organic material called
peat
regions with thick layers of peat = peatlands (3% of Earth’s
surface)
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peat contains 30% of world’s soil carbon
450 billion tons of carbon is stored as peat
Sphagnum does not decay easily – phenolic compounds in its
cell walls
peat – fuel source in northern Europe rather than wood
overharvesting of Sphagnum – could alter CO2 levels
globally
Sphagnum moss
Seedless Vascular Plants
• bryophytes prominent during the first 100 million years of
plant evolution
– but they are not very tall
– rarely over 20 cm in height
• those plants that could achieve heights would have better
access to sunlight, better spore dispersal
• height would mean the need for a transport system for
water and nutrients
• would also need a structural support system
• ferns are example of the evolution of plants that began to
develop height and a vascular system
• fossils of present day vascular plants date back 425 MYA
Seedless Vascular Plants
• 4 major characteristics of vascular plants:
– 1. dominant phase in the alternation of generations life cycle is the
sporophyte
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e.g. ferns – the leafy plant is the sporophyte
the sporophyte becomes the larger and more complex stage of the life cycle
dramatic reduction in gametophyte stage – may be under the soil
sporophyte no longer dependent on the gametophyte for nutrition
– 2. development of vascular tissues – xylem and phloem
• xylem – conduction of water and minerals
– included tracheids – dead, tube-shaped cells for the conduction of water
and minerals up from the roots
– so vascular plants are often referred to as tracheophytes
– water conducting cells contain a phenolic polymer – lignin
– cells are said to be lignified
– this permits vascular plants to grow tall – lignin strengthens the walls
• phloem – conduction of sugars and other nutrients
– living cells
– arranged into tubes for the distribution of sugars, amino acids and other
organic products
Seedless Vascular Plants
• 4 major characteristics:
– 3. development of sporophylls: modified leaves that bear sporangia
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vary in structure
two types: microphyll and megaphyll
e.g. in ferns – megaphylls with clusters of sporangia called sori
e.g. in lycophytes and gymnosperms – microphylls that form cone-like strobili
– most seedless vascular plants are homosporous – one type of sporangium
that produces one type of spore
• this spore produces the two types of gametes = bisexual
– heterosporous species has two types of sporangia that develop into two
types of spores
• megasporangium - megaspore = egg
• microsporangium - microspore - sperm
Seedless Vascular Plants
• 4 major characteristics of vascular plants:
– 4. development of roots and leaves
• rather than rhizoids – the sporophytes of vascular plants have evolved roots
• roots – organs for the anchorage of the plant & absorption of water and nutrients
– resembles the stem tissues of fossilized plants –evolved from them?
• leaves – organs for the increase of vascular surface area to capture more solar energy
– the sporophyte has two types: either megaphylls or microphylls
– megaphylls are larger and have a highly branched vascular system (of veins)
running through them
» greater photosynthetic capacity
– microphylls are spine-like
» supplied by a single, unbranched vein
» appeared to have evolved first
Evolution of Leaves
• evolution of microphylls from clusters of sporangia
• evolution of megaphylls from an accumulation of
branches on a stem
– one branch with overtopping growth
– smaller branches flattened and fused to one another and to the
overtop branch
Seedless Vascular plants
• two clades: Phylum Lycophyta and Phylum Pterophyta
• have modified leaves called sporophylls that bear sporangia
• two types of sporophylls: microphylls and megaphylls
• most seedless vascular plants are homosporous – one type of
sporophyll producing one type of spore that develops into a
bisexual gametophyte
• Phylum Pterophyta – ferns, horsetails and whisk ferns
– the pterophytes are divided by some botanists into separate phyla:
• phylum Sphenophyta – horsetails
• phylum Psilophyta – whisk ferns and relatives
• phylum Pterophyta – ferns
• most recent information consider these groups now to be one
clade
• Phylum Lycophyta – club mosses, spike mosses and quillworts
Phylum Lycophyta
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club mosses, spike mosses and quillworts
1200 species today
NOT true mosses since they have vascular tissue
most ancient line of vascular plants
microphyll line of evolution
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Strobili
(clusters of
sporangia)
Diphasiastrum tristachyum, a club moss
distinct line of evolution that came out of the first land plants
development of leaves from clusters of sporangia
earliest lycophytes formed primitive leaves = enations
enations were small (4 cm) and contained a single trace of
vascular tissue – also very effective at photosynthesis
enations are now called microphylls
“micro” refers to the evolution from small enations not their
size
evolution of true roots – increased the size of the sporophyte
sporangia became clustered into compact cones or strobili
many species evolved heterospory
• modern lycophytes grow on tropical trees as epiphytes –
BUT they are NOT parasites
epiphytic ferns
Phylum Pterophyta
• megaphyll line of evolution
– development of leaves from a branching system of stems
– seen in all seed plants, ferns and arthrophytes (horsetails)
– telome theory: main stem with dichotomously branching
lateral stems
– the lateral branches developed subdivisions – all on one
plant
– the last lateral branches = telomes
– during evolution - tissue grew in between (webbing)
– the telomes also acquired spore-forming ability
– positioning of the branches became very regular and
controlled
– development of phyllotaxy – arrangement of leaves on a
stem
• four basic patterns developed
– some lateral branches became arranged in a
spiral pattern = spiral phyllotaxy
– some phyllotaxy developed an alternating or an
opposite pattern
– others became more complicated (e.g. whorled
pattern)
opposite
alternate
whorled
Phylum Pterophyta
• ferns
– leaves are known as megaphylls
– fern sporophyte is comprised of
underground, horizontal stems
called rhizomes
– from these come vertical shoots
that give rise to large leaves
called fronds divided into
leaflets
– frond grows as the fiddlehead
• leaf primordial cells as they
grow curl inward
– mature frond is called the
megaphyll
– megaphyll is a compound leaf
with a center rachis and multiple
leaflets
– although some fern species – e.g.
staghorn fern – have a simple
leaf structure
Phylum Pterophyta
– gametophyte is very small and shrivels and dies after the
young sporophyte develops
– the diploid sporophyte bears sporangia (singular =
sporangium) clustered under the leaflets in structures called
sori (singular = sorus)
• a sporangium contains spore mother cells (2n)
– the spore mother cells undergo meiosis to produce spores (n)
– these spores are for development of a haploid gametophyte
Annulus and spore dispersal
• http://www.youtube.com/watch?v=xF83pHEx6Q
• the sporangium is stalked with
spring-like devices that disperse the
spores = annulus
• annulus, a row of cells that
bisects the sporangium like a
sturdy spine.
• annulus walls are permeable to
water
• as the sporangium dries,
evaporating water is drawn out
from the annulus, causing the
cells to shrink – this pries the
sporangium open
• the presence of water within
the sporangium propels the
spores out like a catapult
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meiosis of spores within a sporangium
spore release from the sorus and germination
the spore develops into a young gametophyte – bisexual
bisexual gametophyte develops male and female gametogania
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antheridium produces and releases sperm – swims to the egg within the archegonium – fertilization
and development into a zygote
the zygote develops into the diploid sporophyte – emerges from the gametophyte
growth of the sporophyte produces fronds or megaphylls
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male antheridium
female archegonium
young, developing frond is called the fiddlehead
gametophyte disappears
fronds contain sporangia for the production of spores (meiosis)
heterosporous species have megasporangium and microsporangium –production of distinct
spores for male and female gametophyte
almost all fern species are homosporous
http://www.youtube.com/watch
?v=9c9Zi3WFVRc
Key
Haploid (n)
Diploid (2n)
Spore
MEIOSIS
Antheridium
Young
gametophyte
Sporangium
Sperm
Archegonium
Egg
Sporangium
Mature
sporophyte
New
sporophyte
Zygote
Sorus
Gametophyte
Fiddlehead
FERTILIZATION
Fern Life
Cycle
Fern Life Cycle